Science in the spotlight

Measuring the plasma - MAST diagnostics

December 2011

In the UK fusion experiment MAST, ‘mini-stars' or plasmas of over 15 million degrees Celsius are regularly created. For fusion research to reach its goal of commercial electricity generation, scientists need to be able to study what is happening inside the plasma. Measurements of plasma properties such as temperature, density and plasma behaviour such as stability cannot be taken using conventional methods. Developing devices which will measure these properties involves designing complex and innovative diagnostic instruments, often achieved by the sharing of global expertise through long term collaborations with other fusion laboratories and university departments.

On the MAST experiment alone there are over 50 diagnostics, too many to detail here. In the last year, capabilities for studying the MAST plasma have been enhanced by installation of new diagnostics and upgrading of other equipment via collaborative projects. Three such recent projects are outlined here.

A deceptive white box

As you enter the MAST machine area through the south door, straight ahead to the right side of the tokamak is a large four ton white polythene structure. This is the recently upgraded MAST neutron camera, a collaboration between Dr Mikhail Turnyanskiy from CCFE, and Dr Marco Cecconello and a team of three scientists at Uppsala University, Sweden. The large opaque white box contains of a set of detectors that can ‘look' at the plasma through collimated lines of sight and allow the neutron emissivity profile of the plasma to be studied from different angles.

During a fusion experiment on MAST using deuterium gas, with significant neutral beam heating, some fusion neutrons (equivalent to around 100 Watts of power) are emitted and pass through the collimators in the polythene shielding to four detectors. The neutron detectors are able to measure where the neutrons are coming from inside the plasma and distinguish them from other emissions such as x-rays using their energy spectra.

In MAST, neutrons normally only originate from fusion reactions involving fast ions, which are often generated by the neutral beam system. Measurements from the neutrons give us information on the fast ions that formed them. Studying fast ions is one of the main aims of MAST and its future Upgrade as their behaviour helps determine the attractiveness of the Spherical Tokamak concept in the areas of plasma stability, confinement and non inductive current drive. Understanding of the complex nature of interactions between the plasma instabilities and the beam ions requires further investigation and is important for predictions of future larger tokamak facilities such as ITER.

“This project has given Uppsala University a very good opportunity to participate in an international experiment like MAST,” says Dr Cecconello. “We hope we can also collaborate in the future on MAST-Upgrade and by doing so put our expertise together and make the most of these fusion experiments.”

A very sensitive camera

One of the factors which can degrade the energy confinement in plasmas is turbulence or irregular fluctuations. A new diagnostic, the Beam Emission Spectroscopy (BES) turbulence imaging system was installed on MAST in 2010 to measure such phenomena and the associated effect on energy confinement.

The BES system was developed under a collaboration between CCFE and RMKI of the Hungarian Academy of Sciences, Budapest. It detects the fluorescence of the energetic deuterium heating beam, taking two million measurements from each of its 32 spatial channels per second, thereby detecting fluctuations in the local plasma density with very high time and spatial resolution.

“Such images of turbulence in the core of the plasma are only available in a very few fusion experiments worldwide,” explains CCFE's Anthony Field, who designed the system. “Culham has been leading the way in theoretical work on turbulence, but we will now be able to compare our theoretical simulations directly with results from MAST for the first time.”

Keeping the plasma stable within the magnetic field inside a tokamak will be essential for future fusion energy production. Turbulence can threaten this stability, creating irregular fluctuations in the movement of particles from the plasma's core to its edge, which cause unwanted energy losses. Getting a clear picture of the turbulence is therefore essential in understanding and mitigating it for future devices.

The 2D system measures turbulence in MAST by detecting the light emitted when beams of neutral atoms are injected into the plasma to heat it. The diagnostic's very high time resolution allows fusion researchers to map the evolution of turbulent structures at small scales. The four images pictured demonstrate this – showing fluctuations in the density of the plasma at 5 microsecond intervals.

An array of gold antennas

The MAST Electron Bernstein Wave (EBW) emission imaging system consists of an array of 37 antennas, made of RT/Duroid® composite covered with gold, of which up to eight at any one time are used to detect microwaves from the plasma at frequencies not unlike those in a typical kitchen microwave oven. The system is designed to measure the position, intensity and shape of the microwave emissions which occur at the edge of the plasma. The data it collects will allow the calculation of how much electrical current is flowing near the edge of the plasma.

The device was constructed in collaboration between CCFE's Dr Vladimir Shevchenko the University of York's Dr Roddy Vann and PhD student, Simon Freethy, whom they co-supervise. Further developments to the project were made through the expertise of PhD student Billy Huang from Durham University and his work on FPGA technology, as part of the Fusion Doctoral Training network.

Measuring currents in the plasma edge is particularly important because they are thought to determine the dynamics of so-called Edge Localised Modes (ELMs) – sudden eruptive instabilities during which substantial quantities of the total stored plasma energy are lost. Knowledge of such instabilities is crucial for the development of a commercial power plant in which ELMs must be controlled or their effects mitigated. While ELMs are tolerated in present day devices, they constitute a potentially serious heat-load problem for the ITER first wall.

“The aim of the current project is to demonstrate the feasibility of this new technique, which we have now achieved,” says Dr Vann. “There are a lot of key physics questions to answer about ELMs and we are looking forward to using our new imaging system to probe them.”

Accurate measurement of many plasma parameters is what sets apart a world-leading tokamak from other devices.

“MAST is renowned internationally for the excellence of its measuring instruments,” comments Brian Lloyd, MAST Experiments Department Manager. “These exciting new developments keep MAST at the forefront of fusion research and demonstrate the value of close collaboration with our international and UK university partners.”